That unexpected core contained something unusual: sediment layers arranged upside-down compared with the typical pattern.

Turbidites normally show coarse sand at the bottom, with finer silt settling on top.

In this core, fine silt lay beneath coarse sand.

What do the upside-down layers mean?

The reversed layering suggested two events happened in rapid succession.

The researchers concluded the fine silt at the bottom came from a Cascadia earthquake, while the coarse sand above came from subsequent movement on the nearby San Andreas fault.

They call these distinctive structures "doublets."

To pin down ages, the team applied radiocarbon dating to turbidite deposits from that core and from others collected north and south of Cape Mendocino.

Cape Mendocino lies near where the northern San Andreas system and the Cascadia subduction zone converge.

The expanded analysis strengthened the case that doublets record closely spaced ruptures rather than aftershocks or other processes.

What does the "Big One" risk look like now?

Goldfinger said the worst-case scenario for the West Coast may not be a single Cascadia quake.

"We're used to hearing the 'Big One' — Cascadia — being this catastrophic huge thing," he said. "It turns out it's not the worst-case scenario."

If Cascadia and the northern San Andreas ruptured within hours of each other, disaster planners could face a regional emergency rather than a single-city crisis.

"We could expect that an earthquake on one of the faults alone would draw down the resources of the whole country to respond to it," he said. "And if they both went off together, then you've got potentially San Francisco, Portland, Seattle and Vancouver all in an emergency situation in a compressed timeframe."

How rare is fault synchronization?

Scientists have directly documented synchronized ruptures between separate fault systems only once before.

In 2004 and 2005, earthquakes in Sumatra occurred three months apart.

The new Cascadia–San Andreas episodes appear to have been much closer in time — minutes to hours.

What are the study's limits?

• The exact time gap between ancient earthquakes remains uncertain.
• Turbidites can also form from storms or other non-seismic triggers.
• Findings rely on a small number of paired events spread over 1,500 years.
• No written record confirms a simultaneous San Andreas event in 1700.

What's next?

The researchers do not claim that every Cascadia quake sets off the San Andreas.

Instead, they argue the two systems deserve more attention as a coupled hazard.

Co-authors include Ann Morey, Christopher Romsos, and Bran Black of Oregon State's College of Earth, Ocean, and Atmospheric Sciences, plus Jeff Beeson of NOAA Oregon. Other contributors are Maureen Walzcak of the University of Washington, Alexis Vizcaino of Springer Nature Group in Germany, Jason Patton of the California Department of Conservation, and C. Hans Nelson and Julia Gutiérrez-Pastor of the Instituto Andaluz de Ciencias de la Tierra in Spain.

The study appears in the journal Geosphere (DOI: 10.1130/GES02857.1).